PROJECT SUMMARY In addition to intrinsic genetic programs, the development and function of multiple tissues is influenced by extrinsic factors including host-associated microbes. Similarly, host-associated microbes have been linked to neurological disorders including autism-spectrum disorder, multiple sclerosis, Alzheimer?s disease, and Parkinson?s disease. To develop successful therapeutics for these disorders we must understand the roles of extrinsic factors such as the intestinal microbiota in influencing neurodevelopment. Increasing evidence in animal models suggests that the intestinal microbiota modulates social behavior. However, it is still unclear how neuronal characteristics might be influenced by specific microbial strains and products. In initial work, we have demonstrated that the intestinal microbiota is required for social behavior in zebrafish. Additional preliminary results have led us to hypothesize that microbial modulation of social behavior centers on neuronal development in the zebrafish forebrain. The goal of the proposed work is to use the advantages of our gnotobiotic zebrafish model to identify the specific pathways by which microbial strains and products modulate social behavior. With the ability to manipulate and monitor both the intestinal microbiota and host genetics in large numbers of intact adult and juvenile zebrafish, we have a unique opportunity to reveal mechanistic insights that current models of microbial influence in the brain lack. In Aim 1, I will measure social behavior during a developmental time series to determine the critical period during which the intestinal microbiota influences zebrafish behavior. In Aim 2 I will use high resolution microscopy in intact zebrafish to test whether the intestinal microbiota is required for development of neuronal features that influence network function, including neuronal morphology and activity. Finally, in Aim 3 I will use our established gnotobiotic pipeline to identify the particular bacterial strains and their products that mediate microbial influence on social behavior. Insights from these experiments will suggest new avenues for probiotic and antibiotic interventions in multiple neurological disorders by revealing the specific mechanisms that link the intestinal microbiota and development of social behavior.